The invention relates to a method and apparatus for atomizing liquids. In particular, the present invention relates to a method and apparatus for atomizing liquids during normal operating conditions when a high pressure fluid is available for atomization as well as under start-up conditions when only a low pressure gas is provided. The presented embodiment relates to a method and apparatus for atomizing heavy hydrocarbon fuels such as diesel, as part of a fuel reforming process.
Fuel cell technology is an efficient and environmentally friendly power source that has the potential to revolutionize the transportation industry. Unlike fossil fuels that produce undesirable by-products such as sulfur dioxides, nitrogen oxides and carbon monoxide, fuel cells powered by pure hydrogen are virtually emission free. The problem is that although hydrogen exists on Earth in vast amounts it rarely exists in its pure H2 form. Therefore, one of the keys to bringing fuel cells to the mass market (for example use in vehicles) is finding a pure hydrogen source that is both environmentally responsible and economically feasible. Some fuel cell cars are being developed to operate on pure hydrogen (produced at off-site plants) that is pumped directly into the car. The drawback of this approach is that it would require the expenditure of hundreds of billions of dollars to create a new hydrogen distribution network.
Using on-board reformers that convert commercially available hydrogen sources like gasoline and diesel to hydrogen are becoming increasingly attractive. The reformation of hydrocarbon fuels releases CO2 and other gases at substantially lower amounts than existing vehicles. Furthermore, using commercially available fuels as the source of hydrogen would save billions in infrastructure costs.
Catalytic auto-thermal reforming (xe2x80x9cATRxe2x80x9d) is one of the most effective methods of producing hydrogen from heavy hydrocarbons like gasoline and diesel. In ATR, sub-stoichiometric amounts of air partially oxidize the fuel and liberate heat for the endothermic reactions involving steam. During start-up conditions steam is generally not available and hydrogen is produced by partial oxidation of the fuel. During normal operating conditions the hydrocarbon feed stocks are reformed to a hydrogen rich stream by passing the air, steam, and raw fuel over a catalyst bed in the reformer (See, U.S. Pat. No. 6,045,772 issued to Szydlowski et al., issued on Apr. 4, 2000). The actual product yield depends on the catalyst, fuel composition and reactor efficiency. In operation a number of factors influence reactor efficiency including: the amount of mixing between the air, steam and fuel, the method of injection of the fuel-steam-air mixture into the reactor, and the pressure drop of air across the system.
One of the most important factors of ATR efficiency is proper mixing of the fuel, steam and air before contacting the catalyst. Incomplete mixing and gas phase reactions lead to temperature non-uniformities and poor hydrocarbon conversion. Hot spots result if the mixture is locally air rich producing conditions that are favorable for the catalysts to sinter. Coke formation occurs if the mixture is locally fuel rich or lean in H20, decreasing the efficiency of the reactor.
When reforming heavy hydrocarbons, the method of obtaining a well mixed water-fuel-steam spray and injecting it into the reforming reactor is critical. Previous experience with diesel fuel indicates that heavy hydrocarbons need to be injected directly into the ATR as a liquid, since attempts at vaporization generally lead to coke formation. See, U.S. Pat. No. 6,444,179 issued to Sederquist, on Sep. 3, 2002. Optimal efficiency is obtained when the fuel is atomized into droplets of approximately 10 xcexcm or finer. Mechanical atomizers are generally limited to producing droplets that are about 50 xcexcm, which is too large to provide adequate mixing and coke free operation. Therefore, high pressure, gas assisted atomizers are needed to atomize the fuel into a proper droplet size.
From the standpoint of fuel cell efficiency, it is desirable to minimize the pressure drop of the air across the ATR system and specifically across the spray nozzle. The increased air pressure required to overcome a large air side pressure drop has a high energy cost and negatively impacts system efficiency. For this reason the nominal air drop across the nozzle should be restricted to about 1 psi.
Because the power required to pump liquids is much less than that of air, the pressure drops for the atomizing liquid and feed stock can be much higher than the air side drop and are nominally specified to be less than 200 psi.
Steam is the preferable atomizing fluid. However, under ATR start up conditions steam is not available and only air can be used for atomization.
There is a need in the art for a method and apparatus that atomizes heavy hydrocarbon fuels for insertion into an ATR. In particular, there is a need in the art for a method and an apparatus for atomizing heavy hydrocarbons that intimately mixes the air, steam and fuel, produces droplet sizes of around 10 xcexcm during normal conditions, limits the air drop across the nozzle to around 1 psi, and is operable during both normal and start-up conditions.
The three-fluid atomizing nozzle generally comprises a nozzle body and nozzle cap. The nozzle body houses at least three fluid/gas lines including: a feed stock tube (supplying the fluid to be atomized), a high pressure atomization fluid tube (i.e. steam) and a low pressure dispersion fluid tube (i.e. air). The nozzle cap attaches to the nozzle body and forms a mixing chamber where the three fluids mix during normal operation.
Water particularly in the form of steam is the preferred atomizing fluid, however, other fluids capable of atomization could also be used. The feed stock can be any liquid that needs to be atomized. The presented embodiments use air as the dispersion gas, however, other gasses could also be used.
During normal operating conditions the atomizing fluid is fed from the atomizing tube into the atomization cavity of the nozzle body. The atomizing fluid interacts with the feed stock supplied by the parallel feed stock tube. The streams of feed stock liquid and atomizing fluid are oriented in a way to achieve intimate mixing of the two streams. Angling the streams toward each other is one way to ensure proper mixing and to encourage atomization. The flow rates and pressures of the atomizing and feed stock streams should be at sufficient levels to atomize the fluids into a desired droplet size. Multiple atomizing tubes may used to achieve increased atomization.
A nosepiece located at the convergence of the feed stock and atomizing streams straightens out the atomized feed stock jet. In addition, at startup the nosepiece causes the feed stock to film across the sharp edged surface of the nosepiece.
The feed stock and atomizing streams interact and the atomizing fluid atomizes the feed stock. The resulting atomization fluid-feed stock spray passes through the nosepiece aperture and issues into a mixing chamber that is formed when the nozzle cap is attached to the nozzle body.
A dispersion gas is introduced through the dispersion gas tube that runs parallel to the feed stock and atomizing tubes. As the dispersion gas exits the nozzle body it is forced to turn by the nozzle cap that surrounds the nozzle body promoting mixing between the feed stock, high-pressure atomizing fluid (when available) and low-pressure dispersion gas. The geometry of the cap should allow the dispersion fluid to be directed radially into the atomized feedstock spray. The resulting well mixed spray exits an aperture in the nozzle cap producing a spray with desired droplet size. The diameter of the nozzle cap""s aperture determines the extent of mixing between the feed stock, atomizing liquid and dispersion gas.
During start-up conditions (in an ATR) steam is usually not available and atomization must be achieved without the use of a high-pressure atomizing fluid. The three-fluid atomizing nozzle is designed to allow the feed stock to film across the sharp-edged nosepiece. The low velocity dispersion gas radially interacts with this feed stock film atomizing the feedstock into a slightly coarser spray than achieved under normal conditions when the high pressure atomization fluid is available.
The invention involves supplying a stream of feed stock liquid into a atomization chamber with at least one high pressure atomization jet; orienting the high pressure atomization jet(s) and feed stock streams in a way to achieve intimate mixing of the two streams; atomizing the feed stock stream with the atomization fluid from said atomization jet(s) forming droplets containing a well mixed atomizing fluid-feed stock mixture; directing the atomizing fluid-feed stock spray into a mixing chamber; radially inserting a dispersion air into the mixing chamber, forming a well mixed atomizing-feed stock-dispersion spray mixture.
During start-up conditions where a high pressure atomizing fluid is not available, the feed stock forms a film and the dispersion gas radially contacts the feed stock film atomizing the feed stock into droplets.
When used in conjunction with an ATR, the present invention allows the atomization of a fuel source in both normal and start-up conditions. During normal operations the fuel is atomized with a high pressure steam, forming atomized fuel-steam droplets which are subsequently mixed with air allowing for autothermal reformation of the mixture. During start-up (when no steam is available and therefore no high pressure steam for atomization or for use in the autothermal reaction) the fuel forms a film over part of the nozzle and the fuel is atomized by a radially directed low pressure air source, forming atomized fuel-air droplets allowing reformation of the fuel by partial oxidation until enough steam is generated to proceed with normal operation.